The present invention relates generally to a projection optical system, and more particularly to a catadioptric projection optical system that projects an object, such as a single crystal substrate and a glass plate for a liquid crystal display (“LCD”), using a mirror. The present invention is suitable, for example, an immersion exposure apparatus (an immersion lithography exposure system) for exposing the object through a fluid between the projection optical system and the object.
The photolithography technology for manufacturing fine semiconductor devices, such as semiconductor memory and logic circuits, has conventionally employed a reduction projection exposure apparatus that uses a projection optical system to project a circuit pattern of a reticle (or mask) onto a wafer, etc. A more highly integrated and finer semiconductor device (circuit pattern) requires a projection optical system for a better specification and performance. Generally, a shorter wavelength of the exposure light and a higher numerical aperture (“NA”) are effective to improve the resolution. Recently, an optical system with an NA of 1 or higher that utilizes an immersion optical system that fills a space with fluid between a final glass surface (in other words, the lens closet to the wafer) of the projection optical system and the wafer has been proposed, and a higher NA scheme is in progress.
For the exposure light with a short wavelength such as an ArF excimer laser (with a wavelength of approximately 193 nm) and a F2 laser (with a wavelength of approximately 157 nm) for higher resolution, lens materials are limited to quartz and calcium fluoride for reduced transmittance. An optical system that includes only lenses (refracting element) absorbs the large amount of light, and reduces an exposure dose on the wafer, causing a decrease in through put. Moreover, lens's heat absorption and resultant temperature rise disadvantageously fluctuate a focal position, (heat) aberrations, etc. Quartz and calcium fluoride possess similar dispersive powers, and have difficulties in correcting the chromatic aberration. Especially, the lens material can use only calcium fluoride for the exposure wavelength of 157 nm, and correcting the chromatic aberration becomes more difficult. In addition, a lens diameter increases as the NA becomes higher, and causes the increased apparatus cost.
Various proposals that use a mirror (reflecting element) for an optical system have been made to solve the disadvantageous reduced transmittance, difficult corrections to the chromatic aberration and large aperture of the lens. For example, a catadioptric projection optical system that combines a mirror with a lens has been disclosed. See, for example, U.S. Pat. No. 5,650,877 (reference 1), Japanese Patent Applications, Publication Nos. 62-210415 (reference 2), 62-258414 (reference 3), 5-188298 (reference 4), 6-230287 (reference 5), 2-66510 (reference 6), 3-282527 (reference 7), 8-304705 (reference 8), 2000-47114 (reference 9), and 2003-43362 (reference 10).
A projection optical system that includes a reflective optical system for a shorter exposure wavelength and a higher NA needs to correct the chromatic aberration, to maintain a large enough imaging area on an image surface, and to secure a sufficient working distance on the image side with a simple structure. The large enough imaging area on the image surface is advantageous to a scanning exposure apparatus to maintain the throughput, and reduce exposure fluctuations. The sufficient image-side working distance is desirable for an apparatus's auto-focusing system, a wafer-stage's transport system, and the like. The simple structure would simplify a barrel and the like, and facilitate the assembly production.
The optical system disclosed in the reference 1 arranges a Mangin mirror and a refractor in an optical system, and exposes a reticle image onto a wafer. Disadvantageously, this optical system blocks light on a pupil's central part for all the angles of view to be used (hollow illumination), and cannot enlarge an exposure area. An attempt to enlarge the exposure area results in the undesirable expansion of the light blockage on the pupil's central part. In addition, since a refractive surface of the Mangin mirror forms the light splitting surface that halves the light intensity when ever the light passes through its surface, and reduces the light intensity down to about 10% on the image surface (wafer surface).
The references 2 and 3 apply Cassegrain and Schwarzschild mirror systems, and each propose an optical system that has an opening at the center of the mirror to create a hollow illumination to the pupil and to image only the pupil's periphery. However, the hollow illumination on the pupil deteriorates the imaging performance. An attempt to lessen the hollow illumination to the pupil inevitably increases the power of the mirror and enlarges an angle incident upon the mirror. A high NA causes a mirror's diameter to grow remarkably.
According to the optical system disclosed in the references 4 and 5, the deflected optical path complicates an apparatus's configuration. A high NA is structurally difficult because the concave mirror is responsible for most powers in the optical units for imaging an intermediate image onto a final image. Since a lens system located between the concave mirror and the image surface is a reduction system and the magnification has a positive sign, the image-side working distance cannot be sufficiently secured. Since an optical path needs to be split, it is structurally difficult to secure an imaging area width. The large optical system is not suitable for foot-printing.
The references 6 and 7 first split an optical path using by the beam-splitter, and complicate the structure of a lens barrel. They need the beam-splitter with a large diameter and if the beam-splitter is a prism type, a loss of the light intensity is large due to its thickness. A higher NA needs a larger diameter and increases a loss of the light intensity. Use of a flat-plate beam splitter is also problematic, because it causes astigmatism and coma even with axial light. In addition to asymmetrical aberrations due to heat absorptions and aberrations due to characteristic changes on the beam splitting surface, accurate productions of the beam splitter is difficult.
The optical system disclosed in the references 8 to 10 propose a twice-imaging catadioptric optical system for forming an intermediate image once. It includes a first imaging optical system that has a reciprocating optical system (double-pass optical system) which includes concave mirrors to form an intermediate image of a first object (e.g., a reticle), and a second imaging optical system that forms the intermediate image onto a surface of a second object (e.g., a wafer). The optical system of the reference 8 arranges a first plane mirror near the intermediate image for deflecting an optical axis and light near the intermediate image. The deflected optical axis is made approximately parallel to a reticle stage and is deflected once again by a second plane mirror, or an image is formed onto a second object without a second plane mirror. In the optical system of the reference 9, a positive lens refracts light from a first object (e.g., a reticle), and a first plane mirror deflects the optical axis. A second plane mirror in a first imaging optical system again deflects the light reflected by a reciprocating optical system that includes a concave mirror to form an intermediate image. The intermediate image is projected onto a second object (e.g., a wafer) with a second imaging optical system. However, a magnification of the first imaging optical system serves as a reduction system more (corresponding to a paraxial magnification |β1| of about 0.625 of the first imaging optical system). Therefore, the first intermediate image enlarges the NA of the first intermediate image for an object-side NA in the first object by the reduction magnification. As a result, an incident angle range upon the plane mirror increases, and a higher NA scheme as in the immersion etc. encounters a serious problem. In other words, the first imaging optical system that controls a reduction magnification, a higher NA excessively increases the incident angle range upon the plane mirror, and a reflection film on the plane mirror causes a large difference in reflected light's intensity between the p-polarized light and s-polarized light. As a result, a critical dimension (“CD”) difference undesirably increases in an effective image plane. In the optical system of the reference 10, a first plane mirror deflects light from a first object (e.g., a reticle), a second plane mirror deflects the light reflected by a reciprocating optical system that includes a concave mirror, and a positive lens forms an intermediate image. The intermediate image is projected onto a second object (e.g., a wafer) with a second imaging optical system. Thus, a distance from the second plane mirror to the intermediate image becomes long by forming the intermediate image via the positive lens, and a light diameter on the second plane mirror becomes large. Therefore, an influence to a quality of the image projected onto the image surface by a few flaws exited on the reflection surface can be disregarded. Moreover, an asymmetry contribution to an imaging error such as coma generated by heating to the lens is compensated by arranging the positive lens in before and after the middle image and symmetry. However, it is difficult to control reflection film properties because the incident angle upon the plane mirror is large. In other words, the light intensity difference between the p-polarized light and s-polarized light increases by the influence of the reflection film on the plane mirror, and the CD difference in the effective image plane will be increased.
On the other hand, the optical system shown in FIG. 4 of the reference 10 reflects light from a first object (e.g., a reticle) by a reciprocating optical system that includes a concave mirror, deflects light on a return path optical path of the light returned from the concave mirror of the reciprocating optical system in a direction that intersects with light on a forward path optical path of the light traveled to the concave mirror by a first plane mirror, and forms an intermediate image via a lens. The optical system deflects light from the intermediate image by a second plane mirror, and projects onto a second object (e.g., a wafer). However, the optical system only changes an arrangement of the plane mirror, without changing numerical example for the structure of the first embodiment of the reference 10, and the reference 10 does not advert about the influence to the reflection film by the incident angle upon the plane mirror. Moreover, all embodiments have a structure that arranges the positive lens between the first plane mirror and the intermediate image. Therefore, an incident angle range upon the second plane mirror increases by higher NA of the intermediate image, and a design of the reflection film on the mirror and control of forming film are difficult. An arrangement of the optical system becomes difficult by a physical interference between a marginal ray on the forward path of the reciprocating optical system and the lens according a higher NA. In addition, a distance from the first plane mirror to the intermediate image need to long, the light diameter on the first plane mirror becomes large, and light on the forward path is limited. Therefore, it is difficult to secure an enough effective imaging area. A higher first object point to secure the effective imaging area becomes a wide angle, and correcting the aberration becomes difficult. Moreover the chromatic coma aberration by the lens between the first plane mirror and the intermediate image becomes in the same direction as the chromatic coma aberration generated in the second imaging optical system. Therefore, the chromatic coma aberration increases, and it is difficult to obtain a desired imaging performance according a higher NA.